There isn't really great need to differentiate external validity vs generalisability vs transportability, since we now we have a unified framework to handle them all, as in ucla.in/2Jc1kdD. The most important distinction one needs to make is about the disparities
between the study and target populations, i.e., whether such disparities are "man-made" (as in recruiting subjects) or "nature made" (eg age differences).
The interplay between the two is described in ucla.in/2L6yTzE. Still, however we taxonomize these subproblems,
I would be very weary of any theory that does not provide you with playful solutions to at least some toy problems, for example, the three toy problems in Fig. 3 of ucla.in/2N7S0K9. #Bookofwhy @BrownUniversity #kolokotrones @HarvardEpi @harvard_data